It’s quite literally size discrimination. Airlines must be pretty hard-up for money if this is the route they’re going. Note that they wouldn’t be able to go this route PR-wise if the global climate wasn’t so anti-fat (anti-obesity attitudes and measures are popping up nearly everywhere, including Samoa).
I can’t speak to policy measures concerning size discrimination, since I’m an anarchist and in my world the skies would be unregulated (bringing the cost of travel way down, and opening the skies to much more innovative technologies than the same jetplanes we’ve had for 50+ years), and therefore companies would profit off volume, and for places where most people are fat that would mean they would have an interest in catering to fat people instead of doing everything they can to ban them from the skies.
So those are my thoughts, I suppose.